Field of the Invention
The present invention relates generally to an evaporative type cooling system for an internal combustion engine wherein liquid coolant is permitted to boil and the vapor used as a vehicle for removing heat therefrom, and more specifically to such a system which is able to very rapidly vary the temperature of the engine in a manner wherein in addition to permitting the temperature of the engine to be controlled to a level suited for the instant set of operating conditions, permits specific control which enables engine knock to be attenuated and/or obviated without the need to retard the ignition to the degree that engine power output is lost during modes of operation when high power output is in demand.
Description of the Prior Art
In currently used "water cooled" internal combustion engines liquid is forcefully circulated by a water pump, through a cooling circuit including the engine coolant jacket and an air cooled radiator. This type of system encounters the drawback that a large volume of water is required to be circulated between the radiator and the coolant jacket in order to remove the required amount of heat.
Due to the large mass of water inherently required, the warm-up characteristics of the engine are undesirably sluggish. For example, if the temperature difference between the inlet and discharge ports of the coolant jacket is 4 degrees, the amount of heat which 1 Kg of water may effectively remove from the engine under such conditions is 4 Kcal. Accordingly, in the case of an engine having an 1800 cc displacement (by way of example) is operated full throttle, the cooling system is required to remove approximately 4000 Kcal/h. In order to achieve this, the water pump is required to produce a flow rate of approximately 167 liter/min between the coolant jacket and the radiator. This of course undesirably places a parasitic load on the engine which consumes several horsepower.
Further, the large amount of coolant utilized in this type of system renders the possibility of quickly changing the temperature of the coolant to suit the instant set of engine operational conditions such as load and engine speed, completely out of the question.
With this type of cooling system if the the engine is subject to "knocking" the only practical method of dealing with this problem is to retard the ignition timing.
FIG. 2 shows an arrangement disclosed in Japanese Patent Application Second Provisional Publication No. Sho. 57-57608. This arrangement has attempted to vaporize a liquid coolant and use the gaseous form thereof as a vehicle for removing heat from the engine. In this system the radiator 1 and the coolant jacket 2 are in constant and free communication via conduits 3, 4 whereby the coolant which condenses in the radiator 1 is returned to the coolant jacket 2 little by little under the influence of gravity.
This arrangement while eliminating the power consuming coolant circulation pump which plagues the above mentioned arrangement, has suffered from the drawbacks that the radiator, depending on its position with respect to the engine proper, tends to be at least partially filled with liquid coolant. This greatly reduces the surface area through which the gaseous coolant (for example steam) can effectively release its latent heat of vaporization and accordingly condense, and thus has lacked any notable improvement in cooling efficiency.
Further, with this system in order to maintain the pressure within the coolant jacket and radiator at atmospheric level, a gas permeable water shedding filter 5 is arranged as shown, to permit the entry of air into and out of the system.
However, this filter permits gaseous coolant to readily escape from the system, inducing the need for frequent topping up of the coolant level. A further problem with this arrangement has come in that some of the air, which is sucked into the cooling system as the engine cools, tends to dissolve in the water, whereby upon start up of the engine, the dissolved air tends to come out of solution and forms small bubbles in the radiator which adhere to the walls thereof and form an insulating layer. The undissolved air tends to collect in the upper section of the radiator and inhibit the convection-like circulation of the vapor from the cylinder block to the radiator. This of course further deteriorates the performance of the device.
As the interior of this system is maintained constantly at atmospheric pressure via the provision of the air permeable filter 5, this system has lacked the ability to vary the internal pressure of the system which permits the boiling point of the coolant to be modified and thus enable the temperature of the engine to be varied in accordance with the instant set of operating conditions and therefore has not provided any particular solution to the occurence of engine knock.
Further, when the engine is not in use atmospheric air is permitted to contact the upper sections of the interior of the cooling system and induce rapid rusting and the like degradation.
European Patent Application Provisional Publication No. 0 059 423 published on Sept. 8, 1982 discloses another arrangement wherein, liquid coolant in the coolant jacket of the engine, is not forcefully circulated therein and permitted to absorb heat to the point of boiling. The gaseous coolant thus generated is adiabatically compressed in a compressor so as to raise the temperature and pressure thereof and thereafter introduced into a heat exchanger (radiator). After condensing, the coolant is temporarily stored in a reservoir and recycled back into the coolant jacket via a flow control valve.
This arrangement has suffered from the drawback that when the engine is stopped and cools down the coolant vapor condenses and induces sub-atmospheric conditions which tend to induce air to leak into the system. During subsequent engine operation this air tends to be forced by the compressor along with the gaseous coolant into the radiator. Due to the difference in specific gravity, the above mentioned air tends to rise in the hot environment while the coolant which has condensed moves downwardly. The air, due to this inherent tendency to rise, tends to form pockets of air which cause a kind of "embolism" in the radiator and which badly impair the heat exchange ability thereof.
Experiments have shown that the provision of the compressor renders the control of the pressure prevailing in the coolant jacket of the system for the purpose of varying the coolant boiling point with load and/or engine speed, difficult.
U.S. Pat. No. 4,367,699 issued on Jan. 11, 1983 in the name of Evans (see FIG. 3 of the drawings) discloses an engine cooling system wherein the coolant is boiled and the vapor used to remove heat from the engine. This arrangement features a separation tank 6 wherein gaseous and liquid coolant are initially separated. The liquid coolant is fed back to the cylinder block 7 under the influence of gravity while the relatively dry gaseous coolant (steam for example) is condensed in a fan cooled radiator 8.
The temperature of the radiator is controlled by selective energizations of the fan 9 which matches the rate of condensation therein with the engine load and the rate that coolant vapor is generated and in a manner which is sufficient to provide a liquid seal at the bottom of the device. Condensate discharged from the radiator via the above mentioned liquid seal is collected in a small reservoir-like arrangement 10 and pumped back up to the separation tank via a small constantly energized pump 11.
This arrangement, while providing an arrangement via which air can be initially purged to some degree from the system tends to, due to the nature of the arrangement which permits said initial non-condensible matter to be forced out of the system, suffers from rapid loss of coolant when operated at relatively high altitudes. Once the engine cools air is relatively freely admitted back into the system. The provision of the bulky separation tank 6 also renders engine layout difficult.
Further, as the rate of condensation in the consensor is controlled by a temperature sensor (T/S) disposed on or in the condensor per se, in a manner which holds the pressure and temperature within the system essentially constant, temperature variation with load is rendered impossible thus preventing any possibility of engine knock control via temperature variation.
Japanese Patent Application First Provisional Publication No. Sho. 56-32026 (see FIG. 4 of the drawings) discloses an arrangement wherein the structure defining the cylinder head and cylinder liners are covered in a porous layer of ceramic material 12 and wherein coolant is sprayed into the cylinder block from shower-like arrangements 13 located above the cylinder heads 14. The interior of the coolant jacket defined within the engine proper is essentially filled with gaseous coolant during engine operation at which time liquid coolant is constantly sprayed onto the ceramic layers 12.
However, this arrangement has proven totally unsatisfactory in that upon boiling of the liquid coolant absorbed into the ceramic layers, the vapor thus produced and which escapes toward and into the coolant jacket, inhibits the penetration of fresh liquid coolant into the layers and induces the situation wherein rapid overheat, formation of localized hot spots and thermal damage of the ceramic layers 12 and/or engine soon results. Engine knock is actually promoted by this arrangement.
FIG. 5 shows an arrangement which is disclosed in U.S. Pat. No. 4,616,602 issued on Oct. 14, 1986 in the name of Hirano et al. The disclosure of this application is hereby incorporated by reference thereto. For ease of reference the same numerals as used in the above mentioned Patent are also used in FIGS. 5 and 8 of the instant document.
This arrangement has enabled the temperature of the engine to be very rapidly controlled despite external influences such as cold winds etc., by providing a system which actually pumps coolant into and out of the cooling circuit which is hermetically sealed during engine operation, thus enabling the pressure and therefore the boiling point of the coolant to be rapidly modified.
However, under given circumstances despite the fact that engine knock tends to be inherently inhibited by the evaporative cooling, said undesirable phenomenon tends to occur. For example, as shown in FIG. 6 the solid line trace (A) which denotes the theoretical MBT (Minimum spark advance for Best Torque) ignition timing for the instant type of engine, intersects the curved line (B) which denotes the knocking threshold between the engine speeds denoted by N1 and N2.
Further, as shown in FIG. 7 knocking (hatched zone C) only tends to occur above a predetermined load (T) while the engine is operating within the above mentioned engine speed range of N1-N2.
Once knocking is encountered it is necessary to retard the ignition timing significantly from that required for MBT and thus induces the drawback that combustion characteristics and power generation are deteriorated at time when large amounts of power are in demand.